(1) Field of the Invention
This invention relates to a light-emitting element. Particularly usable for emitting a white light.
(2) Related Art Statement
As a light-emitting element for illumination, a normal light source such as a incandescent lamp and fluorescent lamp and also, a light-emitting diode (LED) which is made by stacking plural semiconductor layers are used. Since a white light includes three elementary colors, and can be emitted from the incandescent lamp or fluorescent lamp, the use of the incandescent lamp or fluorescent lamp enables a given object to be observed in full color.
On the other hand, since only a monochromatic light is emitted from the LED, and determined by the structure of the LED, the full color observation can not be realized. Recently, however, a LED to emit a white light are being developed actively. For example, such a white LED as using RGB or using two elementary colors constructed of a blue LED and yellow fluorescent substances coated on the blue LED is fabricated as prototypes.
Moreover, an ultraviolet LED is fabricated and then, a white LED is fabricated as prototypes, using fluorescence from the three elementary color fluorescent substances through the irradiation of an ultraviolet beam from the ultraviolet LED. This technique is based on the principle of a fluorescent lamp, and in this case, the ultraviolet LED is used in place of the mercury discharge inside the fluorescent lamp.
Moreover, since the white LED using RGB requires different LED chips, the cost rises, so that it has difficulty making it practicable for lighting. Then, since the white LED using two elementary colors can not be used for the full color observation because it utilizes only two elementary colors, not three elementary colors. Similarly, since the white LED using the ultraviolet LED requires the fluorescent substances of the three elementary colors, separately, the cost is risen. Then, if a GaN-based blue LED is modified into the ultraviolet LED so as to emit a light beam of shorter wavelength, the luminance efficiency is deteriorated remarkably.
Moreover, since the LED itself emit a light beam by utilizing a pn junction between the semiconductor layers, it has the inherent difficulty due to the pn junction control. Particularly, if such an attempt is made as to fabricate the LED of an AlGaInN-based semiconductor material, it is very difficult to fabricate a p-type semiconductor layer.
It is an object of the present invention to provide a new light-emitting element without the above-mentioned problems.
For achieving the above object, this invention relates to a light-emitting element, including a light-emitting layer including a base layer made of a first nitride semiconductor and plural island-shaped crystal portions made of a second nitride semiconductor, and an irradiation source of electron beam which is disposed so as to be opposite to the light-emitting layer,
whereby electron-electron hole pairs in the light-emitting layer are excited through the irradiation of electron beam from the irradiation source, to generate and emit a light.
For achieving the above object, the inventors had been intensely studied, to obtain a new light-emitting element which is different in principle and structure.
In the light-emitting element according to the present invention, since the plural island-shaped crystal portions are created in the light-emitting layer, given carriers are confined in the island-shaped crystal portions. Therefore, if electron beams are irradiated from the irradiation source, the carriers are combined with given electron holes, effectively, and thus, a light of sufficient intensity can be generated and emitted.
Moreover, the binding energy between each carrier and each electron hole depends on the size of the island-shaped crystal portions to which the carrier and the electron hole belong. Therefore, if the plural island-shaped crystal portions are distributed in the base layer within a given range of size, the binding energies of the carriers and the electron holes are varied, which are combined in their respective island-shaped crystal portions and thus, a light of a given chromaticity or a white light can be generated and emitted. As a result, a light-emitting element to generate and emit a white light, which is difficult to be realized conventionally, can be easily obtained.
A carbon nano-tube may be used as the irradiation source of electron beam, and if the distance between the irradiation source and the light-emitting layer is set to be smaller, the light-emitting element can be illuminated below commercial voltage. Moreover, since the current required to drive the light-emitting element can be set to be extremely small, the electric power consumption can be reduced, compared with a incandescent lamp. Then, since such a harmful substance as mercury is not employed, there is no problem in waste disposal.
The island-shaped crystal portions are formed as follows, for example. That is, the in-plane lattice constant of the second nitride semiconductor to compose each of the island-shaped crystal portions is set larger than the in-plane lattice constant of the first nitride semiconductor to compose the base layer or the in-plane-lattice constant of a given nitride semiconductor to compose a base on which the island-shaped crystal portions are made. In this case, when the second nitride semiconductor is deposited on the base or in the base layer by a MBE method or the like, it may be affected by a compressive stress from the base or the base layer. As a result, the second nitride semiconductor is deposited in dot-like shape, and thus, the isolated island-shaped crystal portions are made.
The base is not an essential component in the present invention, but is used for the sake of convenience, in order to form the light-emitting layer of the light-emitting element according to the present invention. Therefore, the base may be removed by a given thermal treatment or a lapping treatment after the base layer and the island-shaped are formed on the base as mentioned above.